At the time of Margot Fonteyn's memorial service in Westminster Abbey, four months after her death in 1991, I was compiling a Radio 3 programme about her. In setting up interviews with people who had travelled across the globe to be present at the service, I discovered that someone else was pursuing the same path: Meredith Daneman had started researching her biography of Fonteyn. As I tried to convince Margot's friends, relations and colleagues to talk candidly about her on the record, I wondered whether Daneman was having more success.

Her fascinating book proves how persistent she was, and how she won the trust of those who knew the much-loved ballerina best. The difficulty we both found was that Fonteyn's generation (she was born in 1919) did not believe in airing any kind of linen in public. In their view, the world had no need to know about a performer's private life. Dancers, when they talked at all, did not discuss their aches and pains, abortions, affairs, plastic surgery and eating habits. Their colleagues were expected to be similarly discreet, especially if they were members of the Royal Ballet.

Equally frustrating was my interviewees' reluctance to pinpoint what made Fonteyn so special - the most famous ballerina the world over. 'There was no one like her,' they'd gush blandly. 'Margot was, well, Margot. She could make you cry just watching her.' Daneman addresses the problem in her prologue: 'How to put something so visual, so potent with theatrical moment that even film cannot capture it, into plain words? How to explain why it is that when, to a particular strain of music, an ordinary mortal steps forward on one leg, raises the other behind her and lifts her arms above her head, the angels hold their breath?'

Daneman refuses to concede defeat, though her prose can sometimes be as misty-eyed as that of a fan's posting on a balletomane website. She locates Fonteyn's extraordinary on-stage appeal in the woman's personal qualities - her moral as much as physical virtues. When her heroine falters in the choices she makes, Daneman's reproaches are all the more telling for coming from such a sympathetic source. She has set out to understand Margot (as she and her informants call her subject) from the inside, helped by family memoirs, letters and confidences from intimates who no longer saw any point in holding back.

Fonteyn's dark, exotic looks came from her mother's side of the family. Her father, Felix Hookham, was a lower-middle-class Englishman; her mother the illegitimate daughter of a rich Brazilian businessman, Antonio Goncalvez Fontes, and an Irishwoman, Evelyn Acheson. When Daneman met members of the Fontes clan in Brazil, she understood how Latin family pride had made it unthinkable that a young dancer, Peggy Hookham, a remote relative, should adopt their name as a stage pseudonym. She altered it instead to Fonteyn, a surname her elder brother later took as his own as well.

Peggy had been taken to dancing classes throughout her childhood by her ambitious mother, known to the ballet world as BQ or Black Queen, after the formidable leading figure in Ninette de Valois's ballet Checkmate. De Valois accepted the solemn 14-year-old into her Sadler's Wells Ballet School and turned her into a ballerina by the time she was 16. De Valois's strategic sense of what her fledgling company needed meant that Fonteyn was often given starring roles at the expense of other talented dancers. It was she who led the company into the Royal Opera House after the war; she who famously conquered New York as Aurora in The Sleeping Beauty in 1949. The English style of dancing was formed on and by Fonteyn, moulded by Frederick Ashton, the choreographer who made the most of her talents.

Julie Kavanagh's comprehensive 1996 biography of Ashton, Secret Muses, first broke the taboos protecting the raffish - later highly respectable - Royal Ballet. Now Daneman reveals that Fonteyn lost her virginity at 16 and that her next lover, the married company conductor Constant Lambert, boasted about her sexual abilities to Ashton (who passed on the comments with a gusto no biographer could ignore). Ashton's early roles for Fonteyn show her as a sensual being: the seductive Creole girl in Rio Grande, Tiresias the sexual experimenter. Then he froze her in perpetual purity: Chloe, Ondine, Sylvia, in his postwar ballets, are maidens beyond reproach. Not until Rudolf Nureyev burst onto the scene was Fonteyn able to be anything other than virginal.

By then, she had rewritten what Daneman calls her pagan past (Fonteyn would make no mention of lovers in her autobiography) in order to assume the role of the perfect ambassador's wife. She married Roberto (Tito) Arias in 1955 when she was in her mid-forties. De Valois reckoned she had only three years' dancing left in her; younger rivals were waiting impatiently for her to retire. Fonteyn transformed herself into a very grande dame, regarded with almost as much awe as the Queen.

The attractions of Arias, serial adulterer and dodgy Panamanian politician, escaped most of Fonteyn's admirers. Daneman, however, brings fresh insights into the nature of his charm and that of his extended family: Margot and his children by his former wife got on very well together, providing her with a warmth missing from her earlier life. The marriage, though, was already turning sour by the time Nureyev defected from the Soviet Union in 1961.

Fonteyn was initially reluctant to perform with him, mutton dancing with lamb. Tito spelt out the options she faced: 'Get on the bandwagon, or get out'. She obliged, thereby changing the course of her life as well as her career. No longer a fading star, she rose eagerly to the challenges Nureyev set her.

Whether they had a love affair in the sexual sense matters less than their romance on stage. Daneman weighs the views of those who believed they must have slept together and those who thought him too homosexual, she too ladylike. Yet she'd had flings with Roland Petit and Robert Helpmann, among other partners - so why not with highly-sexed Rudolf? The only conclusion is that nobody knew for certain except them. Their personal relationship altered, in any case, over the years, while their performing partnership remained remarkably constant.

Fonteyn kept on dancing long after she should have stopped. She needed to earn money to support her husband, paralysed in an assassination attempt in 1964. Although the last part of her life might have seemed tragic, she embraced her role as Tito's carer wholeheartedly - perhaps too much so for his taste. After she retired at 60, she reinvented herself as a cattle rancher with him in rural Panama. The final photograph in the book shows her, ethereally thin from cancer, flanked by pedigree cows. Shrouded in dust, they echo the ghostly Wilis who claimed Giselle. Daneman has brought Margot, the woman, fully to life in her long-awaited biography.